I have an MP3 player and the mains charger delivers 5V, 2.4A. The manufacturer's "proper" in-car charger has the following specification: Input 12V - 24V DC, Output 5V 2.5A . I recently picked up an in-car charger (cigarette-lighter socket thingy) which can do multiple voltages ... except 5V of course. It does 4.5V and it does 6V - am I going to damage anything by trying to charge or run the MP3 player off either of these voltages? If not, which one should I use? Also, how do I determine which polarity to switch the charger to?
A direct approach to "which charger" would be to measure the actual current and voltage delivered by the manufacturer's charger to the mp3 player under varying load conditions (idle, in use, flashing lights, whatever it does). Then make a resistive dummy load that gives the same current/voltage results with the real charger attached to it and test the multiple output wall-wart to see which setting is closest in voltage for the required current draw.
Somewhat more approximately: multiple output wall-warts are generally not very "stiff." Under light load conditions, the 4.5 V may really be 6 V or more and the 6 V out, closer to 8 or 9 V. Think of the 4.5 V as "about 5 V for light loads" and the 6 V as "about 5 V for heavy loads."
Assuming that the multiple-output wart is rated for at least 2.5 A, I'd start with the 4.5 V setting and see how it goes. Probably (but no guarantees) that will work. If not, bump it up to 6.
Without having measured the actual performance of the manufacturer's device and also the wart, either alternate setting does carry a slight risk, perhaps of not a high enough voltage for the mp3's regulator or of too high a voltage, forcing it to dump excess as heat and possibly damaging the player.
As to which polarity, the manufacturer's charger or the mp3 player should have a diagram like +-Co-- or --Co-+ to indicate what's required. To be sure, go to your closest Radio Shack and get one of their autoranging pocket multimeters and measure it. They're pretty cheap and they do come in handy ...
If the in-car charger you have truly puts out 6 volts at at least 2.5 amps, you can put a 1N5401 diode in series with it and the MP3. That will deliver ~5.3 volts under load. You will need to determine the polarity of things - use your meter to determine which is + and -.
If you have doubts about the in-car charger, forget the whole thing.
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